In general, an automatic transmission mounted in a vehicle includes a plurality of planetary gear sets (planetary gear mechanisms), and a plurality of frictional engagement elements (particularly hydraulic frictional engagement elements) such as clutches and brakes. These frictional engagement elements are selectively engaged by hydraulic control, for example, to change power transmission paths through the planetary gear sets, thereby providing a plurality of forward gears and reverse gears (usually one reverse gear).
For example, PATENT DOCUMENT 1 discloses an automatic transmission, which includes three planetary gear sets, and five frictional engagement elements. Two of the frictional engagement elements are engaged to provide one of six forward gears and one reverse gear.
In recent years, there are attempts to increase the number of gears of the automatic transmission to improve the fuel efficiency. One of the methods of increasing the number of gears is to increase the number of planetary gear sets. For example, Patent Document 2 discloses an automatic transmission including four planetary gear sets and five frictional engagement elements to provide eight forward gears and one reverse gear.